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Neutron dosimetry onboard aircraft using superheated emulsions
941
Neutron dosimetry onboard aircraft using superheated emulsions M. Hajek a , T. Berger a , N. Vana a,b , B. Mukherjee c a Atominstitute of the Austrian Universities, Stadionallee 2, A-1020 Vienna, Austria b Austrian Society for Aerospace Medicine, Lustkandlgasse 52/3, A-1090 Vienna, Austria c Australian Nuclear Society and Technology Organization, PMB 1, Menai, NSW 2234, Australia
Roughly half of the radiation exposure at subsonic aviation altitudes is caused by neutrons in a wide energy range. Superheated emulsions, also known as bubble detectors, represent a relatively novel technology within neutron dosimetry. The capabilities of these instruments, which were calibrated in the CERN-EU High-Energy Reference Field, as accurate and easyto-handle in-flight neutron dosemeters are demonstrated by applications on both north-bound and trans-equatorial flight routes. The results are compared with simultaneous thermoluminescent dosemeter measurements.
1. Introduction Because of the high and energy-dependent biological effectiveness of neutron radiation, the exact determination of the neutron energy spectrum and the neutron dose has gained increased importance within radiation protection during the last decade. Neutrons can be a serious source not only around nuclear fission and fusion reactors or high-energy accelerators, but also constitute a major component of the natural radiation environment at high altitudes. Roughly half of the radiation exposure of aircrew personnel is caused by cosmic ray-induced neutrons in a wide energy range extending up to the GeV region. The exposure of aircraft crew to cosmic radiation has, therefore, been included as occupational exposure in a directive of the European Council [1]. However, neutrons are effective not only in causing biological hazards, but may as well affect aircraft electronics, which contains several gigabytes of semiconductor memory, by producing so-called single event effects (SEE), e.g. bit-flips. In this respect, accurate neutron dosimetry contributes to a better understanding of the radiation load on aircrew personnel and frequent flyers and may as well stimulate the improvement of reliability and availability of both appropriate shielding materials and avionics hardware. The precise assessment of the ambient dose equivalent delivered by neutrons requires either RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07117-7
© 2005 Elsevier Ltd. All rights reserved.
942
M. Hajek et al.
the determination of the neutron energy spectrum in-flight or the application of a dosemeter system characterised by a response function that matches the fluence-to-dose equivalent conversion curve.
2. Instruments and methods 2.1. Superheated emulsions Superheated emulsions, also known as bubble detectors, have been used in radiation detection, dosimetry and spectrometry for slightly over two decades and, thus, represent a relatively novel technology. The detection principle is based on the use of superheated halocarbon and/or hydrocarbon droplets suspended in a compliant tissue equivalent material, e.g., a soft polymer or an aqueous gel [2]. Charged particles liberated by radiation interactions nucleate the phase transition of the superheated liquid and generate detectable bubbles. Although general agreement exists on the qualitative description of the nucleation process, a universally accepted quantitative theory which incorporates all aspects of the phenomenon is still elusive. A reasonable mechanism by which nucleation arises in superheated liquids
Fig. 1. Energy response of the BD-100R type bubble detector. Reprinted from H. Ing, Radiat. Measur. 33 (2001) 275.
Neutron dosimetry onboard aircraft using superheated emulsions
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is proposed in the so-called “thermal spike theory” [3]. This theory assumes that ionising radiation produces highly localised hot regions or thermal spikes within the liquid, which literally explodes into bubbles in the evaporation process. The physical processes responsible for the production of bubbles are believed to be similar to those responsible for producing radiation damage in solids. For the purposes of the present work, three detectors available from Bubble Technology Industries, Inc., under the trade name BD-100R were selected. They consist of proprietary formulations of superheated liquids dispersed in a firm elastic polymer matrix. Since the host medium is firm, the bubbles do not migrate but remain at the sites of formation and can be counted by the naked eye. The BD-100R detectors possess a temperature-dependent nominal sensitivity of 2.2 bubbles µSv−1 . However, the instrument’s response as shown in Fig. 1 sharply drops at energies exceeding 20 MeV and, therefore, does not follow the fluence-todose equivalent conversion curve anymore. 2.2. Thermoluminescent dosemeters Lithium fluoride thermoluminescent dosemeters (TLDs) are commonly employed for the determination of absorbed dose. The application of the 6 Li-enriched commercially available detector type TLD-600 in combination with the 7 Li-enriched TLD-700 opens up interesting possibilities for neutron dosimetry. TLD-600 and TLD-700 detector crystals possess the unique advantage that they show, in principle, almost identical response to photons and charged particles, but very different response to thermal (and epithermal) neutrons. This property stems from the 6 Li(n, α)3 H reaction, which dominates the TLD-600 response. The reaction cross section is 943.2 barn at an energy of 0.0253 eV, compared with a total neutron cross section value of 14.7 barn for 7 Li at the same energy, as can be inferred from Fig. 2. In order to ascertain the thermal neutron-induced TL-signal, the gamma-equivalent absorbed doses measured with TLD-600 and TLD-700 are subtracted. Therefore, by arranging TLD-600 and TLD-700 dosemeters in pair, the net thermal neutron dose may be determined.
Fig. 2. Total neutron cross sections of different lithium isotopes.
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M. Hajek et al.
3. Calibration The properties of both techniques, bubble detectors and the pair-method with TLDs, constitute certain limitations with respect to in-flight neutron dosimetric applications. Bubble detectors of the type BD-100R count only bubbles in the energy region between approximately 200 keV and 11 MeV. The pair-method is usually restricted to thermal neutrons. However, the overall neutron dose equivalent onboard aircraft may be determined if the calibration of the systems was performed in a reference field with a spectral composition similar to the neutron field at aviation altitudes. The neutron energy spectrum was measured in-flight by means of a passive Bonner sphere spectrometer [4] and revealed to relative maxima around 1 and 85 MeV. The CERN-EU High-Energy Reference Field (CERF) simulates the atmospheric neutron spectrum and represents an excellent calibration facility, since the relative thermal fraction of the neutron fluence is practically the same as for the cosmic ray-induced neutron spectrum in the Earth’s atmosphere [5,6]. The CERF calibration data can, therefore, be applied for measurements onboard aircraft. The CERF facility is installed in the H6 secondary beam line of the Super Proton Synchrotron (SPS), located in the northern experimental area on the French site of CERN. The stray radiation field originates from a mixed beam of positively charged hadrons with momenta of 120 GeV c−1 (35% protons, 61% pions and 4% kaons) incident on a cylindrical copper target (7 cm diameter × 50 cm length), which is located under a 80 cm-thick concrete shield. The roof shield produces an almost uniform radiation field over an area of 2 × 2 m2 , divided into 16 reference positions of 50 × 50 cm2 for which the spectral fluence rates of the different particles, i.e. mainly neutrons, but also photons, electrons, muons, pions and protons, are simulated to a good level of detail by the Monte Carlo-code FLUKA. By adjusting the beam intensity on the target one can vary the dose equivalent rate at the reference positions, typically in the range from 5 to 600 µSv h−1 . The results of FLUKA-calculations of the neutron energy spectra at CERF [7] are compared in Figs. 3 and 4 with calculations of the neutron spectrum at a depth in the atmosphere of 200 g cm−2 , corresponding to an altitude of
Fig. 3. Calculated neutron fluence spectra.
Neutron dosimetry onboard aircraft using superheated emulsions
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Fig. 4. Calculated neutron ambient dose equivalent fluence spectra.
11.9 km (39 000 ft), by S. Roesler et al. [8]. The data are presented as fluence, ϕ, or ambient dose equivalent, H ∗ (10), in an energy bin, divided by the logarithmic bin width. The area of a bin is proportional to the fluence or ambient dose equivalent contribution to the total fluence or ambient dose equivalent, which is normalised to unity. The respective conversion coefficients H ∗ (10)/ϕ have been calculated by B.R.L. Siebert et al. [8] and extended by D. Bartlett [6] towards higher energies. Behind the concrete shield, the neutron radiation field reproduces the major components, albeit in different proportions, of the neutron radiation field in aircraft produced by cosmic radiation. However, this does not directly imply any consequences for the neutron dose. Appropriate conversion coefficients [9] can be used to calculate ambient dose equivalent values from the neutron fluence spectra normalised to unity total fluence, both for the CERF and the cosmic ray-induced neutron component at an altitude of 11.9 km. The ratio between the two values, H ∗ (10)CERF /H ∗ (10)11.9 km , is between 1.02 and 1.05 for the reference exposure locations on top of the concrete shield. From that, it can be concluded that the actual neutron ambient dose equivalent measured with an instrument that was calibrated in the CERF will be underestimated by 2 to 5%. For practically every device, this should be within the statistical uncertainty of the measurement itself. This indicates the applicability of the CERF calibration for in-flight neutron measurements. The calibration factors for the bubble detectors, which are identified by their serial number, are given in Table 1. The values are already corrected for the ambient temperature of 25 ◦ C during the irradiation and are, therefore, valid for room temperature of 20 ◦ C. In order to allow the nucleated bubbles to grow to a sufficient size, they were counted about 24 hours after the exposure by three independent persons. After recompression, the detector tubes were stored in air-tight aluminium containers at about 20 ◦ C. Since the detector lifetime is restricted to about one and a half year, the relative response was further checked pre- and post-flight by means of an exposure in the field of an 241 Am9 Be neutron source. In order to obtain a reasonable statistics, the pair-method with TLDs subtracts the averaged TL-signals of at least six TLD-600 and six TLD-700 chips, respectively. The calibration factor is batch-dependent and was evaluated as 1.347 counts µSv−1 for the employed crystals.
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M. Hajek et al. Table 1 Calibration of three BD-100R bubble detectors in the CERN-EU High-Energy Reference Field (the calibration ambient dose equivalent was 28.4 µSv) Serial #
Average number of counted bubbles
Calibration factor (bubbles µSv−1 )
109376 114451 118277
44.9 ± 3.7 41.4 ± 1.7 41.2 ± 1.7
1.58 ± 0.13 1.46 ± 0.06 1.45 ± 0.06
Table 2 Neutron ambient dose equivalents measured on the return flights Vienna–Sydney and Vienna–Tokyo Flight route
Neutron ambient dose equivalent (µSv)
Vienna–Sydney
Bubble detectors: TLDs Bubble detectors: TLDs:
Vienna–Tokyo
27.7 ± 2.7 25.3 ± 3.0 29.7 ± 5.2 26.0 ± 2.7
Neutron ambient dose equivalent rate (µSv h−1 ) Bubble detectors: TLDs: Bubble detectors: TLDs:
0.7 ± 0.1 0.7 ± 0.1 1.1 ± 0.2 0.9 ± 0.1
4. Results and discussion In-flight measurements using the three bubble detectors and a mixed TLD-600/TLD-700 package were conducted for the return flights Vienna–Sydney and Vienna–Tokyo at the end of October and the beginning of November 2001. Whereas the route Vienna–Sydney constitutes a typical trans-equatorial flight, Vienna–Tokyo was flown mostly at mid latitudes but certainly closer to the pole than the other route. The ambient temperature was estimated by means of a temperature indicator, which is located under the piston screw of the bubble detector tubes. The evaluation process for both dosemeter systems was the same as during the calibration procedure. The resulting neutron ambient dose equivalent values are summarised in Table 2. The data from bubble detectors and TLDs agree well within the statistical uncertainty. It is obvious that the neutron dose rate (0.7 µSv h−1 for Vienna–Sydney and 1.0 µSv h−1 for Vienna–Tokyo) increases with increasing geomagnetic latitude. This is addressed to lower geomagnetic shielding of primary cosmic ray-particles at pole-near regions. It shall further be stated that the investigations were performed approximately one year after the maximum of the 23rd solar activity cycle, so that the dose rates may be regarded as minimum values.
5. Conclusions In terms of the biologically relevant dose equivalent, neutron radiation constitutes the dominant component of the radiation environment at aviation altitudes. The conducted experiments confirmed that superheated emulsions and thermoluminescent dosemeters of the types TLD600 and TLD-700 arranged in pair are reliable monitoring instruments for the neutron dose equivalent onboard aircraft. The extension of the so-called pair-method represents a novel approach in neutron dosimetry. Both systems are passive devices, i.e. they consume no power
Neutron dosimetry onboard aircraft using superheated emulsions
947
and do not interfere with aircraft electronics by the emission of electromagnetic radiation. They are, furthermore, comparably easy-to-handle with detection limits sufficiently low to match the requirements of routine applications. However, with certain expenditure statistical uncertainties of about 15% and below are achievable.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Council Directive 96/29/Euratom of May 13, 1996, Official J. Eur. Commun. Ser. L 159 (29.6.1996) 1. F. d’Errico, Nucl. Instrum. Methods B 184 (2001) 229. H. Ing, R.A. Noulty, T.D. McLean, Radiat. Measur. 27 (1997) 1. M. Hajek, T. Berger, W. Schöner, N. Vana, in: Proc. ANS Biennial RSPD Topical Meeting, Santa Fe, 2000, published on CD-ROM. A. Mitaroff, M. Silari, CERN TIS-2001-006-RP-PP, 2001. M. Hajek, T. Berger, W. Schöner, N. Vana, Trans. Am. Nucl. Soc. 83 (2000) 263. T. Otto, private communication, 1999. S. Roesler, private communication, 2001. B.R.L. Siebert, H. Schuhmacher, Radiat. Prot. Dosim. 58 (1995) 177.
Neutron dosimetry onboard aircraft using superheated emulsions M. Hajek a , T. Berger a , N. Vana a,b , B. Mukherjee c a Atominstitute of the Austrian Universities, Stadionallee 2, A-1020 Vienna, Austria b Austrian Society for Aerospace Medicine, Lustkandlgasse 52/3, A-1090 Vienna, Austria c Australian Nuclear Society and Technology Organization, PMB 1, Menai, NSW 2234, Australia
Roughly half of the radiation exposure at subsonic aviation altitudes is caused by neutrons in a wide energy range. Superheated emulsions, also known as bubble detectors, represent a relatively novel technology within neutron dosimetry. The capabilities of these instruments, which were calibrated in the CERN-EU High-Energy Reference Field, as accurate and easyto-handle in-flight neutron dosemeters are demonstrated by applications on both north-bound and trans-equatorial flight routes. The results are compared with simultaneous thermoluminescent dosemeter measurements.
1. Introduction Because of the high and energy-dependent biological effectiveness of neutron radiation, the exact determination of the neutron energy spectrum and the neutron dose has gained increased importance within radiation protection during the last decade. Neutrons can be a serious source not only around nuclear fission and fusion reactors or high-energy accelerators, but also constitute a major component of the natural radiation environment at high altitudes. Roughly half of the radiation exposure of aircrew personnel is caused by cosmic ray-induced neutrons in a wide energy range extending up to the GeV region. The exposure of aircraft crew to cosmic radiation has, therefore, been included as occupational exposure in a directive of the European Council [1]. However, neutrons are effective not only in causing biological hazards, but may as well affect aircraft electronics, which contains several gigabytes of semiconductor memory, by producing so-called single event effects (SEE), e.g. bit-flips. In this respect, accurate neutron dosimetry contributes to a better understanding of the radiation load on aircrew personnel and frequent flyers and may as well stimulate the improvement of reliability and availability of both appropriate shielding materials and avionics hardware. The precise assessment of the ambient dose equivalent delivered by neutrons requires either RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07117-7
© 2005 Elsevier Ltd. All rights reserved.
942
M. Hajek et al.
the determination of the neutron energy spectrum in-flight or the application of a dosemeter system characterised by a response function that matches the fluence-to-dose equivalent conversion curve.
2. Instruments and methods 2.1. Superheated emulsions Superheated emulsions, also known as bubble detectors, have been used in radiation detection, dosimetry and spectrometry for slightly over two decades and, thus, represent a relatively novel technology. The detection principle is based on the use of superheated halocarbon and/or hydrocarbon droplets suspended in a compliant tissue equivalent material, e.g., a soft polymer or an aqueous gel [2]. Charged particles liberated by radiation interactions nucleate the phase transition of the superheated liquid and generate detectable bubbles. Although general agreement exists on the qualitative description of the nucleation process, a universally accepted quantitative theory which incorporates all aspects of the phenomenon is still elusive. A reasonable mechanism by which nucleation arises in superheated liquids
Fig. 1. Energy response of the BD-100R type bubble detector. Reprinted from H. Ing, Radiat. Measur. 33 (2001) 275.
Neutron dosimetry onboard aircraft using superheated emulsions
943
is proposed in the so-called “thermal spike theory” [3]. This theory assumes that ionising radiation produces highly localised hot regions or thermal spikes within the liquid, which literally explodes into bubbles in the evaporation process. The physical processes responsible for the production of bubbles are believed to be similar to those responsible for producing radiation damage in solids. For the purposes of the present work, three detectors available from Bubble Technology Industries, Inc., under the trade name BD-100R were selected. They consist of proprietary formulations of superheated liquids dispersed in a firm elastic polymer matrix. Since the host medium is firm, the bubbles do not migrate but remain at the sites of formation and can be counted by the naked eye. The BD-100R detectors possess a temperature-dependent nominal sensitivity of 2.2 bubbles µSv−1 . However, the instrument’s response as shown in Fig. 1 sharply drops at energies exceeding 20 MeV and, therefore, does not follow the fluence-todose equivalent conversion curve anymore. 2.2. Thermoluminescent dosemeters Lithium fluoride thermoluminescent dosemeters (TLDs) are commonly employed for the determination of absorbed dose. The application of the 6 Li-enriched commercially available detector type TLD-600 in combination with the 7 Li-enriched TLD-700 opens up interesting possibilities for neutron dosimetry. TLD-600 and TLD-700 detector crystals possess the unique advantage that they show, in principle, almost identical response to photons and charged particles, but very different response to thermal (and epithermal) neutrons. This property stems from the 6 Li(n, α)3 H reaction, which dominates the TLD-600 response. The reaction cross section is 943.2 barn at an energy of 0.0253 eV, compared with a total neutron cross section value of 14.7 barn for 7 Li at the same energy, as can be inferred from Fig. 2. In order to ascertain the thermal neutron-induced TL-signal, the gamma-equivalent absorbed doses measured with TLD-600 and TLD-700 are subtracted. Therefore, by arranging TLD-600 and TLD-700 dosemeters in pair, the net thermal neutron dose may be determined.
Fig. 2. Total neutron cross sections of different lithium isotopes.
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M. Hajek et al.
3. Calibration The properties of both techniques, bubble detectors and the pair-method with TLDs, constitute certain limitations with respect to in-flight neutron dosimetric applications. Bubble detectors of the type BD-100R count only bubbles in the energy region between approximately 200 keV and 11 MeV. The pair-method is usually restricted to thermal neutrons. However, the overall neutron dose equivalent onboard aircraft may be determined if the calibration of the systems was performed in a reference field with a spectral composition similar to the neutron field at aviation altitudes. The neutron energy spectrum was measured in-flight by means of a passive Bonner sphere spectrometer [4] and revealed to relative maxima around 1 and 85 MeV. The CERN-EU High-Energy Reference Field (CERF) simulates the atmospheric neutron spectrum and represents an excellent calibration facility, since the relative thermal fraction of the neutron fluence is practically the same as for the cosmic ray-induced neutron spectrum in the Earth’s atmosphere [5,6]. The CERF calibration data can, therefore, be applied for measurements onboard aircraft. The CERF facility is installed in the H6 secondary beam line of the Super Proton Synchrotron (SPS), located in the northern experimental area on the French site of CERN. The stray radiation field originates from a mixed beam of positively charged hadrons with momenta of 120 GeV c−1 (35% protons, 61% pions and 4% kaons) incident on a cylindrical copper target (7 cm diameter × 50 cm length), which is located under a 80 cm-thick concrete shield. The roof shield produces an almost uniform radiation field over an area of 2 × 2 m2 , divided into 16 reference positions of 50 × 50 cm2 for which the spectral fluence rates of the different particles, i.e. mainly neutrons, but also photons, electrons, muons, pions and protons, are simulated to a good level of detail by the Monte Carlo-code FLUKA. By adjusting the beam intensity on the target one can vary the dose equivalent rate at the reference positions, typically in the range from 5 to 600 µSv h−1 . The results of FLUKA-calculations of the neutron energy spectra at CERF [7] are compared in Figs. 3 and 4 with calculations of the neutron spectrum at a depth in the atmosphere of 200 g cm−2 , corresponding to an altitude of
Fig. 3. Calculated neutron fluence spectra.
Neutron dosimetry onboard aircraft using superheated emulsions
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Fig. 4. Calculated neutron ambient dose equivalent fluence spectra.
11.9 km (39 000 ft), by S. Roesler et al. [8]. The data are presented as fluence, ϕ, or ambient dose equivalent, H ∗ (10), in an energy bin, divided by the logarithmic bin width. The area of a bin is proportional to the fluence or ambient dose equivalent contribution to the total fluence or ambient dose equivalent, which is normalised to unity. The respective conversion coefficients H ∗ (10)/ϕ have been calculated by B.R.L. Siebert et al. [8] and extended by D. Bartlett [6] towards higher energies. Behind the concrete shield, the neutron radiation field reproduces the major components, albeit in different proportions, of the neutron radiation field in aircraft produced by cosmic radiation. However, this does not directly imply any consequences for the neutron dose. Appropriate conversion coefficients [9] can be used to calculate ambient dose equivalent values from the neutron fluence spectra normalised to unity total fluence, both for the CERF and the cosmic ray-induced neutron component at an altitude of 11.9 km. The ratio between the two values, H ∗ (10)CERF /H ∗ (10)11.9 km , is between 1.02 and 1.05 for the reference exposure locations on top of the concrete shield. From that, it can be concluded that the actual neutron ambient dose equivalent measured with an instrument that was calibrated in the CERF will be underestimated by 2 to 5%. For practically every device, this should be within the statistical uncertainty of the measurement itself. This indicates the applicability of the CERF calibration for in-flight neutron measurements. The calibration factors for the bubble detectors, which are identified by their serial number, are given in Table 1. The values are already corrected for the ambient temperature of 25 ◦ C during the irradiation and are, therefore, valid for room temperature of 20 ◦ C. In order to allow the nucleated bubbles to grow to a sufficient size, they were counted about 24 hours after the exposure by three independent persons. After recompression, the detector tubes were stored in air-tight aluminium containers at about 20 ◦ C. Since the detector lifetime is restricted to about one and a half year, the relative response was further checked pre- and post-flight by means of an exposure in the field of an 241 Am9 Be neutron source. In order to obtain a reasonable statistics, the pair-method with TLDs subtracts the averaged TL-signals of at least six TLD-600 and six TLD-700 chips, respectively. The calibration factor is batch-dependent and was evaluated as 1.347 counts µSv−1 for the employed crystals.
946
M. Hajek et al. Table 1 Calibration of three BD-100R bubble detectors in the CERN-EU High-Energy Reference Field (the calibration ambient dose equivalent was 28.4 µSv) Serial #
Average number of counted bubbles
Calibration factor (bubbles µSv−1 )
109376 114451 118277
44.9 ± 3.7 41.4 ± 1.7 41.2 ± 1.7
1.58 ± 0.13 1.46 ± 0.06 1.45 ± 0.06
Table 2 Neutron ambient dose equivalents measured on the return flights Vienna–Sydney and Vienna–Tokyo Flight route
Neutron ambient dose equivalent (µSv)
Vienna–Sydney
Bubble detectors: TLDs Bubble detectors: TLDs:
Vienna–Tokyo
27.7 ± 2.7 25.3 ± 3.0 29.7 ± 5.2 26.0 ± 2.7
Neutron ambient dose equivalent rate (µSv h−1 ) Bubble detectors: TLDs: Bubble detectors: TLDs:
0.7 ± 0.1 0.7 ± 0.1 1.1 ± 0.2 0.9 ± 0.1
4. Results and discussion In-flight measurements using the three bubble detectors and a mixed TLD-600/TLD-700 package were conducted for the return flights Vienna–Sydney and Vienna–Tokyo at the end of October and the beginning of November 2001. Whereas the route Vienna–Sydney constitutes a typical trans-equatorial flight, Vienna–Tokyo was flown mostly at mid latitudes but certainly closer to the pole than the other route. The ambient temperature was estimated by means of a temperature indicator, which is located under the piston screw of the bubble detector tubes. The evaluation process for both dosemeter systems was the same as during the calibration procedure. The resulting neutron ambient dose equivalent values are summarised in Table 2. The data from bubble detectors and TLDs agree well within the statistical uncertainty. It is obvious that the neutron dose rate (0.7 µSv h−1 for Vienna–Sydney and 1.0 µSv h−1 for Vienna–Tokyo) increases with increasing geomagnetic latitude. This is addressed to lower geomagnetic shielding of primary cosmic ray-particles at pole-near regions. It shall further be stated that the investigations were performed approximately one year after the maximum of the 23rd solar activity cycle, so that the dose rates may be regarded as minimum values.
5. Conclusions In terms of the biologically relevant dose equivalent, neutron radiation constitutes the dominant component of the radiation environment at aviation altitudes. The conducted experiments confirmed that superheated emulsions and thermoluminescent dosemeters of the types TLD600 and TLD-700 arranged in pair are reliable monitoring instruments for the neutron dose equivalent onboard aircraft. The extension of the so-called pair-method represents a novel approach in neutron dosimetry. Both systems are passive devices, i.e. they consume no power
Neutron dosimetry onboard aircraft using superheated emulsions
947
and do not interfere with aircraft electronics by the emission of electromagnetic radiation. They are, furthermore, comparably easy-to-handle with detection limits sufficiently low to match the requirements of routine applications. However, with certain expenditure statistical uncertainties of about 15% and below are achievable.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Council Directive 96/29/Euratom of May 13, 1996, Official J. Eur. Commun. Ser. L 159 (29.6.1996) 1. F. d’Errico, Nucl. Instrum. Methods B 184 (2001) 229. H. Ing, R.A. Noulty, T.D. McLean, Radiat. Measur. 27 (1997) 1. M. Hajek, T. Berger, W. Schöner, N. Vana, in: Proc. ANS Biennial RSPD Topical Meeting, Santa Fe, 2000, published on CD-ROM. A. Mitaroff, M. Silari, CERN TIS-2001-006-RP-PP, 2001. M. Hajek, T. Berger, W. Schöner, N. Vana, Trans. Am. Nucl. Soc. 83 (2000) 263. T. Otto, private communication, 1999. S. Roesler, private communication, 2001. B.R.L. Siebert, H. Schuhmacher, Radiat. Prot. Dosim. 58 (1995) 177.